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Our transcription: When erosion eventually strips away the several kilometers of rock covering the batholith, the massive plutonic rock remains as an elongate mountain range. Plutonic rocks are composed of a tightly intergrown mass of crystal which is usually more resistant to weathering than the sedimentary and metamorphic rocks which surround the magmatic intrusion. Geologists use batholiths to try to understand the tectonic history of the Earth's crust. The Sierra Nevada batholith here in Eastern California is an excellent example. The hundreds of individual plutons which make up this batholithic mountain range represent a series of magmatic intrusions that began about 200 million years ago and continued over the next 120 million years. Most of this plutonic rock is chemically very similar to andesite, a volcanic rock that's found forming in continental volcanic arcs, such as the Andes. These arcs exist because oceanic crust is being subducted beneath the edge of a continent. Partial melting of the down going rocks generates the andesitic magma which rises in bouyant plumes to feed the volcanoes or to become part of the batholith below. So the overall structure and composition of the Sierran plutonic rocks tells us that this is a deeply eroded magmatic arc which formed along an ancient subduction margin.
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